The field of the invention is that of motor vehicles. More specifically, the invention relates to flexible axles for motor vehicles.
It will be recalled that a flexible axle is the term generally given to an axle that has been designed to form a torsion element between two wheels.
Conventionally, a flexible axle has comprised two longitudinal arms each bearing a support for mounting a wheel and which are connected by a transverse connecting element known as a crossbrace or profile section.
During the process of designing an axle there are two parameters which, amongst others, are given consideration when assessing the quality of the axle. These are bending and torsion.
The principle of flexible axles means that good bending stiffness can be allied with relative torsional flexibility. In general, it is through the geometry of the cross section of the crossbrace, by way of its bending and torsional moments of inertia, that the desired compromise between bending stiffness and (relative) torsional flexibility is reached.
The cross sections chosen for producing crossbraces made of steel (or any other isotropic material) are often V-shaped, U-shaped, or L-shaped because these types of geometry display an advantageous relationship between bending inertia and torsional inertia.
The last few years have seen the flexible axle technique increasingly spread to the middle and lower end of the automotive construction spectrum thanks to the many advantages they display, these including an excellent compromise between the features they can present and the architecture, and the fact that they can be produced economically chiefly by use of all-welded construction.
These advantages have led the designers of ground contact systems toward constant technological developments in their latest versions. This is because flexible axles do suffer from a certain number of limitations including a delicate compromise between longitudinal and transverse stiffnesses and an endurance which is governed by the durability of each of their component parts which are subjected to significant elastic deformations.
The connecting element, or crossbrace, is one of the most difficult components to develop, particularly from a durability standpoint. Aside from the integrity of the crossbrace body, the regions of abutment against the arms, these abutment regions generally being welded, are particularly highly stressed and development engineers have to give this area a great deal of attention in order to avoid premature breakage of the connection.
Various types of crossbrace and various ways of mounting these between the longitudinal arms of the axle are known.
In a first known technique illustrated by FIGS. 1a, 1b and 1c, the crossbrace 10 has a V-shaped cross section over the entire length of the crossbrace. Furthermore, the height of the flanks of the crossbrace increases at the ends of the crossbrace so as to increase the footprint of the crossbrace on the arms 20.
In a second technique illustrated by FIGS. 2a, 2b and 2c, the crossbrace 10 is bent in the YZ plane (and is therefore generally termed a “banana” beam crossbrace). A crossbrace such as this, of open section with the opening facing toward the rear once mounted, has cutouts at its ends into which the arms 20 are butted. In this state of the art these arms are generally square and this leads to not insignificant additional costs because of the need to resort to certain bending, squashing, etc. operations.
In a third technique illustrated in FIGS. 3a, 3b and 3c, the crossbrace 10 is bent in the YZ plane and in addition has an evolving cross section which starts out with a V-shape in the central region of the crossbrace and ends up in a trapezoidal shape at the ends of the crossbrace.
In this instance it is found that, sooner or later depending on how the vehicle is used, the welded connections between the crossbrace and the longitudinal arms begin to degrade.
Now, an analysis of these phenomena has led to the observation that the weld beads at the crossbrace/arm interface are working in a “peeling” mode, this type of loading corresponding to a known weakness of welded seams which arises in particular out of the orientation of their metallographic structure as they cool.
Another axle technique has been proposed in the prior art, and this is illustrated in FIGS. 4a, 4b, 4c and 4d. 
According to this technique, the crossbrace 1 once mounted has an open section with the opening facing downward, this tending to raise the center of torsion of the axle and giving it better elasto-kinematic behavior.
As can be seen in FIGS. 4a, 4b, 4c and 4d, the crossbrace 10 is positioned substantially at right angles to the longitudinal arm 20, the end of the crossbrace 10 being designed to espouse the shape of the arm 20.
To achieve this, the crossbrace laterally has two abutment portions 11 the shape of which corresponds to the shape (generally cylindrical) of the arm. These abutment portions 11 are conventionally extended in such a way that the crossbrace covers the top of the arm 20.
FIG. 4d clearly shows that a connecting portion 14 connects the two abutment portions of the crossbrace, forming a corner 111 with each of these.
With this type of solution, it is regularly found that the welds are highly loaded at the ends of the seam, and more especially at the corners 111 of the crossbrace 1. This may cause cracks to appear in the weld, these cracks ultimately breaking the welded seam into two. In some instances, these cracks even spread into the axle arm.
Clearly it will be appreciated that this can cause significant damage to the axle, it being possible for this to have repercussions on the level of safety offered by the vehicle equipped with the axle in question.
In any event, it is desirable to eliminate or, at the very least to limit, the aforementioned deterioration.
In addition, when significant loadings are exerted on the abutment regions, particularly as a result of successive bendings of the crossbrace, the corners 111 tend to puncture the wall of the longitudinal arm. In other words, these corners 111, under the effect of vibrations and jolts transmitted by the vehicle, experience micro-movements toward the inside of the arm and ultimately hammer (in the manner of a center punch) the wall of the arm.
This repeated hammering ultimately leads to cracking of the wall of the arm and/or to peeling of the welded seam.
To remedy this situation, one solution might be to increase the wall thickness of the longitudinal arms in order to increase their strength.
However, this would have the consequence of increasing the weight of the longitudinal arms.
In order to limit this hammering phenomenon, one technique has been proposed by the Applicant Company to introduce relative flexibility into the ends of the crossbrace. This is achieved by, for example at each end of the crossbrace, producing a cutout. However, this technique entails performing a step of machining or cutting out the crossbrace before the crossbrace undergoes its pressing operation, and this adds a further step to the axle manufacturing schedule and tends to increase its cost.